Nijmegen Breakage Syndrome (NBS) is a rare autosomal recessive disorder characterized by microcephaly, growth retardation, immunodeficiency, and an increased incidence of cancer, e.g., hematopoietic malignancy (van der Burgt et al., 1996; and Weemaes et al., 1981). At the cellular level, NBS is characterized by cell cycle defects and radiation sensitivity. The clinical features of NBS overlap to some extent with those of ataxia telangiectasia (AT); thus it has been described as an AT variant syndrome (Saar et al., 1997). However, NBS is genetically distinct. The two syndromes are clinically distinguishable in that NBS patients do not exhibit neurological abnormalities, telangiectasia or increased α-feto protein levels observed in AT patients (reviewed in Shiloh, 1997).
Nonetheless, NBS and AT exhibit remarkably similar phenotypes at the cellular level, suggesting that the corresponding gene products function in the same pathway. Unlinked non-complementation of chromosome instability was observed in heterokaryons of AT and NBS fibroblasts, leading to speculation that the respective gene products are physically associated (Stumm et al., 1997). Cells from both NBS and AT patients show increased sensitivity to ionizing radiation (IR) as well as increased levels of spontaneous and induced chromosomal fragility. In addition, NBS and AT cells fail to induce p53 at the G1/S checkpoint, and fail to suppress DNA synthesis in response to ionizing radiation (radioresistant DNA synthesis) (Jongmans et al., 1997; Perez-Vera et al., 1997; Sullivan et al., 1997; Taalman et al., 1983; and Young and Painter, 1989) (AT reviewed in Hoekstra, 1997; and Shiloh, 1997). Together, these data suggest that the AT gene product, ATM, is a component of, or functions in close proximity to, the primary sensor of DNA damage.
Accordingly, AT phenotypes can be explained by the failure to signal the presence of DNA damage. Hence, IR sensitivity in AT cells is generally attributed to defects in the cellular DNA damage response. However, some data suggest that DNA repair functions in AT cells may also be affected (Blocher et al., 1991; Cornforth and Bedford, 1985; Murnane, 1995; and Pandita and Hittelman, 1992). Consistent with this notion, cells established from AT patients exhibit increased rates of intrachromosomal DNA recombination (Meyn, 1993).
DNA damage includes double strand breaks in the DNA. These breaks in DNA are repaired by the double strand break (DSB) repair complex. The human DSB complex, also referred to as the “hMre11/hRad50 complex”, was shown to consist of five proteins: hMre11, hRad50, and three additional proteins of 95 kDa, 200 kDa, and 400 kDa (Dolganov et al., 1996; Petrini et al., 1995). The phenotypic features of yeast lacking the counterparts to hMre11 and hRad50, i.e., Scmre11 and Scrad50 mutants, include hyperrecombination, sensitivity to DNA damaging agents, and DNA repair deficiency (Ajimura et al., 1993; and Game, 1993). These features are reminiscent of chromosomal instability syndromes such as AT, NBS, Bloom syndrome and others (Ellis, 1997; Fukuchi et al., 1989; Gatti et al., 1991; Meyn, 1995; and van der Burgt et al., 1996). The conservation of Mre11 and Rad50 functions predicts that similar phenotypic outcomes would result from mutations in humans that affect the hMre11/hRad50 protein complex. However, deficiencies in hMre11 or hRad50 have not been associated with any known chromosomal instability syndromes. Recent attempts to create a null mre11 mutant in murine embryonic stem cells indicate that the gene is essential, suggesting that spontaneous homozygous null mutants of hMRE11 and hRAD50 will not be found in the human population (Xiao and Weaver, 1997).
Thus, there is a need to isolate and characterize genes encoding double strand DNA repair proteins.